1. Field of the Invention
The present invention relates to improvements in MLS cylinder head gaskets for automotive internal combustion engines. More particularly, the invention relates to gaskets having improved stopper layers to provide enhanced sealing about their combustion apertures.
2. Description of the Prior Art
Those skilled in the art will appreciate the difficulty involved in maintaining a high quality seal between cylinder apertures of an internal combustion engine and a cylinder head fastened to engine block. In recent years, multiple-layered steel (MLS) cylinder head gaskets have become a preferred design choice, wherein all (typically at least three) gasket layers have been formed of steel. Beaded exterior layers have generally been fabricated of a 301 stainless steel, a relatively robust metal with a commensurately high spring rate, for meeting requisite performance requirements over the useful life of the gaskets. The center layer, also called a “spacer” layer, has generally been formed of less expensive metals such as 409 stainless steel, or in some cases even zinc-plated, and other low carbon steels, for meeting less rigorous requirements.
The gasket areas immediately adjacent the circumference of engine cylinder bore apertures are subject to considerably greater stresses for assuring proper sealing than areas of the gasket radially remote from the apertures. To meet the greater stress requirements at the aperture areas, so-called stopper layers have been employed in areas that circumferentially surround each cylinder aperture boundary. The stopper layers are designed to provide increased sealing pressures around areas immediately adjacent the combustion apertures. In some cases the stoppers have been formed of extra layers of metal, consisting either of layers folded over or under primary sealing layers. In other cases, the stoppers have been formed as layers separately provided, e.g., discrete annular rings positioned about the aperture boundaries.
Radially outwardly of the stopper layer, a secondary seal is normally provided. The secondary seal is usually formed as an active spring seal defined by an embossed bead in a primary external metal sealing layer. Two of such embossed beads are normally provided in two separate mirror image external layers that sandwich the center or spacer layer. In some cases, the embossed beads employed in such duplicate mirror image primary sealing layers have been prone to cracking at the boundaries of the beads, particularly where hard metal stopper layers have been employed. The stopper layer dynamics can create stresses on the beads that exceed normal bead deflection stress capability. Resultant cold working and bead cracking are issues seriously detrimental to both the performance and longevity of gaskets that otherwise provide reliable combustion gas sealing media.
To the extent that trends in the design of MLS gaskets have been toward reductions in numbers of layers used, and particularly in cases where such layers cannot feasibly be reduced, non-metallic stoppers have been employed. Such non-metallic stopper structures have generally been formed as part of, and hence integral to, a central unitary metal spacer layer. Many have been employed in combination with metallic bead structures, often deposited by screen-printing in the troughs or undersides of such beads. The employment of screen-printed, non-metallic beads has, however, been associated with higher expense, and in many cases with less than satisfactory results.